Shot used for blast processing

ABSTRACT

The present disclosure relates to a shot used for blast processing, the shot being made of an iron-based alloy containing C: 0.20 to 0.50% by mass, Si: 0.50 to 1.10% by mass, and Mn: 0.50 to 1.15% by mass as additive elements, in which a mass ratio of C to Si is 0.30 to 0.75, a mass ratio of C to Mn is 0.30 to 0.75, and a mass ratio of Si to Mn is 0.70 to 1.60, and a Vickers hardness of the shot is HV 400 to 800.

TECHNICAL FIELD

The present disclosure relates to a shot used for blast processing.

BACKGROUND ART

Shot blasting is used for shake-out of castings after casting, removing burrs of metal products, removing scale such as rust, and the like. The shot blasting is a processing method of projecting particles called shots toward a product to be processed. As the shots, iron-based particles are used in many cases.

Patent Literature 1 discloses a shot which contains C: 0.80 to 1.10% by mass, Si: 0.50 to 1.00% by mass, Mn: 0.50 to 1.00% by mass, and Cr: 0.10 to 0.30% by mass as additive elements and the balance of Fe (including unavoidable impurities). Since the shot is repeatedly used until the shot is worn to a size not suitable for shot blasting, there is a demand for a shot with less wear (long life time).

Further, the hardness of the shot is selected according to the physical properties of a workpiece or the purpose of shot blasting. There is a demand for establishment of a method for producing shots having a wide variety of hardnesses.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Publication No. S53-75156

SUMMARY OF INVENTION Technical Problem

The present disclosure is made in view of the above circumstances, and an object thereof is to provide shots having a long life time since blast cleaning efficiency is hardly impaired and having a wide variety of hardnesses.

Solution to Problem

A shot according to an embodiment of the present disclosure is a shot used for blast processing. The shot is made of an iron-based alloy containing C: 0.20 to 0.50% by mass, Si: 0.50 to 1.10% by mass, and Mn: 0.50 to 1.15% by mass as additive elements. Further, in the shot, a mass ratio of C to Si (C/Si) is 0.30 to 0.75, a mass ratio of C to Mn (C/Mn) is 0.30 to 0.75, and a mass ratio of Si to Mn (Si/Mn) is 0.70 to 1.60. Further, a Vickers hardness of the shot is HV 400 to 800 (defined in JIS Z 2244:2009).

In an embodiment, the total content of C, Si, and Mn (C+Si+Mn) may be 1.80 to 2.40% by mass.

In an embodiment, the iron-based alloy may further contain 0.30 to 1.0% by mass of at least one element selected from the group consisting of Cr, Ni, Cu, Mo, Al, B, V, Nb, and Ti.

In an embodiment, the shot may be substantially formed from a tempered martensite phase.

In an embodiment, the number of particles having blowholes may be 5% or less of the entire shot.

In an embodiment, when a length of the particle in a longitudinal direction is designated as L and a maximum diameter in a direction perpendicular to the longitudinal direction is designated as S, the number of the particles in which L/S is 2.0 or more may be 5% or less of the entire shot.

Advantageous Effects of Invention

According to an embodiment of the present disclosure, there is provided shots having a long life time since blast cleaning efficiency is hardly impaired and having a wide variety of hardnesses.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows results obtained by observing cross-sections of respective shots of Example 1 and Comparative Example with a scanning electron microscope.

DESCRIPTION OF EMBODIMENTS

A shot according to an embodiment of the present disclosure is a shot made of a four-component iron-based alloy containing C, Si, and Mn as additive elements. Alternatively, the shot according to an embodiment of the present disclosure can also be a shot containing particles made of an iron-based alloy, in which the iron-based alloy contains C, Si, and Mn as additive elements. The shot (iron-based alloy) may contain other unavoidable impurities.

Hereinafter, a shot of an embodiment will be described in detail using the case of producing a shot by a water atomization method as an example. Incidentally, in the following description, % refers to % by mass unless otherwise specified.

<Melting Step>

Raw materials (Fe, C, Si, Mn, and the like), which are weighed to have a predetermined composition, for the shot are put into a melting furnace and melted by heating at 1600° C. to 1750° C. to obtain a molten metal.

Fe is an element that becomes a base of the shot.

C is an element that affects hardness. When the concentration of C is increased, the shot is hardened so that the blast cleaning capacity is increased, but toughness is decreased in proportion to the concentration of C. A decrease in toughness leads to a decrease in life time. Taking into consideration of hardness and life time required for the shot, the content of C in the iron-based alloy is 0.20 to 0.50% by mass, and may be 0.30 to 0.45% by mass or 0.35 to 0.45% by mass.

Si has an effect of removing oxygen in a raw material molten metal. Oxygen contained in the raw material molten metal inhibits the spheroidizing of particles at the time of granulation by an atomization method. When the concentration of Si is high, the deoxidation effect is increased so that spherical particles are easily obtained, but toughness is decreased in proportion to the concentration of Si. Further, by removing oxygen in the raw material molten metal, internal defects called blowholes can be reduced. C also has a deoxidation effect, but taking into consideration of life time, it is difficult to add C in such an amount that the deoxidation effect is sufficiently obtained. Taking into consideration of deoxidation effect and life time, the content of Si in the iron-based alloy is 0.50 to 1.10% by mass, and may be 0.55 to 1.05% by mass or 0.60 to 1.00% by mass.

Mn has an effect of enhancing the quenchability of the granulated particles and an effect of shifting a pearlite nose in a TTT curve to the right side to reduce a critical cooling rate. By performing quenching, the hardness required for a shot can be obtained. The structure of the particles is transformed by quenching, but at this time, the structure is required to be a structure having homogenous and fine crystalline grains. Quenching conditions are adjusted so as to obtain such a structure, but in the case of fine particles like shots, delicate adjustment of the quenching conditions is necessary. By adding Mn into the alloy, allowable values of the quenching conditions are widened, and the conditions are easily adjusted. According to this, a structure having homogenous and fine (for example, 2 to 10 μm) crystalline grains is easily introduced over the entire granulated particle. As a result, particles excellent in toughness are obtained so that the life time of the shot is enhanced. However, since Mn is an expensive metal, when the concentration thereof is increased, the production cost of shots is increased. Taking into consideration of quenching effect and cost, the content of Mn in the iron-based alloy is 0.50 to 1.15% by mass, and may be 0.55 to 1.00% by mass or 0.60 to 0.95% by mass.

Further, when the contents of C, Si, and Mn in the iron-based alloy are designated as a % by mass, b % by mass, and c % by mass, respectively, taking into consideration of the synergetic effect of each of C, Si, and Mn as additive elements, each mass ratio or the like is as follows.

a/b (the mass ratio of C to Si) is 0.30 to 0.75, and may be 0.35 to 0.60 or 0.40 to 0.50.

a/c (the mass ratio of C to Mn) is 0.30 to 0.75, and may be 0.35 to 0.60 or 0.40 to 0.50.

b/c (the mass ratio of Si to Mn) is 0.70 to 1.60, and may be 0.80 to 1.40 or 0.90 to 1.20.

a+b+c (the total content of C, Si, and Mn) is preferably 1.80 to 2.40% by mass, and may be 1.85 to 2.25% by mass or 1.90 to 2.10% by mass.

The iron-based alloy may further contain at least one element selected from the group consisting of Cr, Ni, Cu, Mo, Al, B, V, Nb, and Ti. These other additive elements are added for the purpose of complementing the effects of Si and Mn, and the following effects are obtained by addition thereof. However, when the concentration of these other additive elements is too high, problems such as a decrease in hardness, a decrease in life time, and insufficiency of spheroidizing tend to occur. Therefore, the content of these other additive elements in the iron-based alloy (the total content in the case of adding a plurality of elements) may be adjusted to 0.30 to 1.0% by mass.

Cr has an effect of enhancing quenchability and an effect of shifting a pearlite nose in a TTT curve to the right side to reduce a critical cooling rate. When the concentration of Cr is too low, these effects are not sufficiently obtained; on the other hand, when the concentration thereof is too high, the toughness of the shot tends to be decreased. Taking into consideration of these points, the content of Cr in the iron-based alloy may be adjusted to 0.3 to 1.0% by mass.

Ni and Cu have an effect of enhancing quenchability and life time while suppressing spheroidizing inhibition by oxygen. When the concentration of Ni and Cu is too low, these effects are not sufficiently obtained; on the other hand, when the concentration thereof is too high, the toughness tends to be decreased. Taking into consideration of these points, the total content of Ni and Cu in the iron-based alloy may be adjusted to 0.4 to 1.0% by mass. Incidentally, since expression of the above effect is more slightly excellent in Ni than in Cu but Ni is an expensive material, a compositional ratio of Ni and Cu is determined in consideration of the effect and cost.

Mo has an effect of reducing a variation in structure or hardness for each particle, an effect of enhancing quenchability, and an effect of shifting a pearlite nose in a TTT curve to the right side to reduce a critical cooling rate. When the concentration of Mo is too low, these effects tend not to be sufficiently obtained. In addition, since Mo is an expensive material, when the concentration of Mo is too high, a disadvantage caused by an increase in cost is larger than an advantage obtained by those effects. Taking into consideration of these points, the content of Mo in the iron-based alloy may be adjusted to 0.1 to 0.3% by mass.

Al has an effect of removing oxygen in the raw material molten metal to accelerate the spheroidizing of the particles and an effect of reducing blowholes. When the concentration of Al is too low, these effects are not sufficiently obtained; on the other hand, when the concentration thereof is too high, spheroidizing tends to be inhibited. Taking into consideration of these points, the content of Al in the iron-based alloy may be adjusted to 0.04 to 0.12% by mass.

B has an effect of enhancing quenchability and an effect of enhancing life time. When the concentration of B is too low, these effects are not sufficiently obtained; on the other hand, when the concentration thereof is too high, life time tends to be decreased. Taking into consideration of these points, the content of B in the iron-based alloy may be adjusted to 0.01 to 0.05% by mass.

V, Nb, and Ti have an effect of enhancing life time. When the concentration of these elements is too low, the effect is not sufficiently obtained; on the other hand, when the concentration thereof is too high, life time tends to be decreased. Taking into consideration of these points, the total content of V, Nb, and Ti in the iron-based alloy may be adjusted to 0.05 to 0.5% by mass.

<Granulation Step>

The raw material molten metal is dropped from a nozzle at a bottom portion of a dissolution bath, and high-pressure water is sprayed toward this molten metal to thereby obtain a granulated product (spherical body).

Oxygen in the raw material molten metal becomes a factor that inhibits the spheroidizing of the particles at the time of granulation. The shot of an embodiment is formed from a molten metal containing Si as a raw material. Since oxygen in the raw material molten metal is removed by Si, the spheroidizing of the particles can be accelerated by using such a raw material.

Further, oxygen in the raw material molten metal causes blowholes. The blowholes are generated when oxygen in the raw material molten metal is not discharged in air at the time of coagulation of a raw material and is embedded as air bubbles inside the particles. By using Si as a raw material, oxygen in the raw material molten metal is removed, and thus blowholes can be reduced.

<Quenching Step>

Since C is contained in the granulated product produced as described above, the granulated product is harder than in the case of Fe. However, for using the granulated product as a shot, it is necessary to further enhance hardness. The granulated product produced in the granulation step is dried by a rotary kiln or the like, then heated at 800° C. to 900° C., held for about 1 hour, and then dropped in water to thereby perform quenching. According to this, the hardness of the granulated product can be increased.

Since Mn is contained in the granulated product, quenchability is improved. Further, in the granulated product, a pearlite nose in a TTT curve is shifted to the right side so that a critical cooling rate is reduced. Further, at the time of performing quenching, the structure of the granulated product becomes finer so that toughness is enhanced. That is, such a granulated product can be used as a shot with less wear (long life time).

<Tempering Step>

The granulated product undergoing the quenching step is heated at 300° C. to 600° C. for about 0.5 to 2.0 hours and then is gradually cooled to thereby perform tempering. According to this, the granulated product can be adjusted to have a desired hardness, and the toughness, which has been decreased by the quenching step, of the granulated product can be enhanced.

Through the quenching step and the tempering step, a fine and uniform structure is introduced into the granulated product. In particular, the structure of the particles contained in the shot of an embodiment is mainly formed substantially from a tempered martensite phase. The crystalline grain size in the tempered martensite phase is about 0.5 to 10 μm. The particles having such a structure have high toughness even when an impulsive load is repeatedly applied at the time of blast processing, and thus wear thereof is suppressed.

<Classification Step>

The granulated product after the quenching step is sieved using a vibration sieve or the like. According to this, particles having a predetermined diameter are classified.

<Recovery Step>

Through a step of examining the shape, the hardness, and the like of the classified particles, a shot containing particles having a predetermined diameter is obtained.

A Vickers hardness of the shot (particle) of an embodiment is HV 400 to 800. Incidentally, from the viewpoint of blast processability and life time, HV may be 400 to 650 or 400 to 500. The standard deviation of HV can be set to HV 50 or less. Since the shot of an embodiment has a specific composition, the shot has a sufficient hardness as the shot and an extremely small variation in hardness for each particle. When a variation in hardness is extremely small, finished quality at the time of blast processing is stabilized.

In the shot of an embodiment, when a length of the particle in a longitudinal direction is designated as L and a maximum diameter in a direction perpendicular to the longitudinal direction is designated as S, the number of the particles in which L/S is 2.0 or more may be 5% or less of the entire shot. Since the shot of an embodiment has a specific composition, a large number of particles uniformly spheroidized are contained and finished quality at the time of blast processing is stabilized. The number of the particles may be 1% or less or 0.1% or less of the entire shot.

In the shot of an embodiment, the number of particles having blowholes may be 5% or less of the entire shot. Herein, the particles having blowholes indicate particles in which the area ratio of air bubbles in the cross-section is 10% or more of the area of the cross-section and the wall surfaces of the air bubbles are smooth. The blowholes become a starting point at which the shot is damaged at the time of blast processing. Since the shot of an embodiment has a specific composition, the number of particles having blowholes is extremely small and the shot has a long life time. The number of the particles may be 3% or less or 1% or less of the entire shot.

In the shot of an embodiment, the number of particles having cracks may be 5% or less of the entire shot. Herein, the particles having cracks indicate particles in which the length of cracks in the cross-section is three times or more the width of cracks and the length of cracks is the length of 20% or more of the minimum diameter in the cross-section. The cracks become a starting point at which the shot is damaged at the time of blast processing. Since the shot of an embodiment has a specific composition, the number of particles having cracks is extremely small and the shot has a long life time. The number of the particles may be 3% or less or 1% or less of the entire shot.

In the shot of an embodiment, the average particle size of the particles to be contained may be 0.1 to 1.5 mm. When the average particle size is within the above range, the life time of the shot tends to be increased. Herein, the average particle size is a value measured by sieving using a standard sieve of JIS Z 8801. The average particle size may be 0.1 to 1.0 mm, 0.15 to 0.75 mm, or 0.2 to 0.45 mm.

The apparent density of the shot of an embodiment may be 7.45 g/cm³ or more. According to this, collision energy to a product to be processed at the time of blast processing is easily obtained so that the product to be processed is easily subjected to sufficient blast cleaning.

EXAMPLES

Next, test results for confirming the effects of the shot of an embodiment will be described.

First, various shots made of an iron-based alloy having a blending ratio of Table 1 were produced using a water atomization method. X in Table 1 represents the content (total content) of an additive element selected from the group consisting of Cr, Ni, Cu, Mo, Al, B, V, Nb, and Ti.

The obtained particles were classified to obtain shots having desired particle sizes (ϕ0.3 mm, ϕ0.6 mm, and ϕ1.0 mm). Incidentally, the shots having respective particle sizes were prepared as described below.

ϕ0.3 mm: Those which passed through a sieve of 0.425 mm and remained on a sieve of 0.355 mm

ϕ0.6 mm: Those which passed through a sieve of 0.710 mm and remained on a sieve of 0.600 mm

ϕ1.0 mm: Those which passed through a sieve of 1.180 mm and remained on a sieve of 1.000 mm

(1) Measurement of Vickers Hardness

The particles of the shots were embedded in a resin, and then polishing was performed so that the center of the cross-section was exposed to the surface. The Vickers hardness for 10 shots was measured according to the aforementioned standard (JIS Z 2244:2009), and then an average value thereof was regarded as the hardness of the shot. The results are shown in Table 2.

(2) Measurement of Apparent Density

The apparent density was measured according to JIS Z 0311:2004. Specifically, about 10 g of the shot was charged in a pycnometer (volumetric capacity: 50 ml) and then the mass was measured. Next, distilled water was charged in the pycnometer, air bubbles inside thereof were removed, and then the mass was measured. The apparent density was calculated from these masses. This operation was performed two times, and an average value thereof was regarded as the apparent density of the shot. The results are shown in Table 2.

(3) Verification of Defects (Blowholes)

The particles of the shots were embedded in a resin, and then polishing was performed so that the center of the cross-section was exposed to the surface. The cross-section for 100 shots was observed by a projector, the number of shots in which the aforementioned blowholes are present as defects was counted, and then the ratio thereof was calculated. The results are shown in Table 2.

(4) Verification of Sphericity

The particles of the shots were spread on a glass flat plate, and then the length L of the particle in the longitudinal direction and the maximum diameter S in a direction perpendicular to the longitudinal direction for 100 shots were observed with a microscope. Then, the number of particles in which L/S is 2.0 or more was counted, and the ratio thereof (particle shape percent defective) was calculated. The results are shown in Table 2.

(5) Evaluation of Life Time

100 g of the produced shots were put in a life time test device (manufactured by Ervin Industries: The Test Ervin Machine) and projected toward a steel stock (HRC65) at a projection speed of 60 m/s. The shots after projection were recovered, the recovered shots were classified with a sieve (0.300 mm, 0.500 mm, or 0.850 mm), and the weight of the shots remaining on the sieve was weighed. This operation was repeated until the shots remaining on the sieve became 10 g, a curve showing the relation between the number of collisions and the remaining rate of the shots, which had been obtained by this test, was integrated, and this numerical value was regarded as a value of life time. The results are shown in Table 2.

Comparative Example is a shot having a conventional composition. It was found that the shots of Examples 1 to 10 have a longer life time than the shot of Comparative Example.

TABLE 1 Condition Particle Compositional ratio [wt %] size C Si Mn X [mm] Example 1 0.21 0.62 0.63 0.00 0.3 Example 2 0.48 0.85 0.85 0.00 0.3 Example 3 0.35 0.62 0.85 0.00 0.3 Example 4 0.35 1.09 0.85 0.00 0.3 Example 5 0.35 0.85 0.61 0.00 0.3 Example 6 0.35 0.85 1.15 0.00 0.3 Example 7 0.35 0.85 0.85 0.31 0.3 Example 8 0.35 0.85 0.85 0.97 0.3 Example 9 0.35 0.85 0.85 0.00 0.6 Example 10 0.35 0.85 0.85 0.00 1.0 Comparative 0.92 0.79 0.81 0.00 0.3 Example

TABLE 2 Result Particle shape Apparent Percent percent Hardness density defective defective Life time [HV] [g/cm³] [%] [%] [cycle] Example 1 412 7.60 2.8 2.8 4,360 Example 2 487 7.60 2.2 2.1 4,530 Example 3 462 7.63 1.1 1.8 4,730 Example 4 461 7.58 2.4 1.9 4,890 Example 5 441 7.66 1.2 2.5 4,630 Example 6 468 7.63 1.2 1.4 4,820 Example 7 457 7.60 0.8 2.7 5,130 Example 8 470 7.60 0.5 2.5 5,020 Example 9 453 7.63 1.7 2.8 3,500 Example 10 462 7.63 2.5 4.2 3,210 Comparative 466 7.58 1.9 3.9 3,170 Example

Further, the cross-sections of respective shots of Example 1 and Comparative Example were observed with a scanning electron microscope. As shown in FIG. 1, in the shot of Example, the structure mainly formed from the tempered martensite phase that became finer was observed; on the other hand, in the shot of Comparative Example, the structure mainly formed from the martensite phase that did not become finer was observed.

INDUSTRIAL APPLICABILITY

Since the shot of an embodiment has hardness necessary for blast cleaning and a long life time, the industrial value of the shot is extremely large. This shot can be used in any of blast processing.

The water atomization method has been described as an example of the method for producing the shot of an embodiment, but another method such as a gas atomization method or a disk atomization method may be employed. 

1: A shot used for blast processing, the shot being made of an iron-based alloy containing C: 0.20 to 0.50% by mass, Si: 0.50 to 1.10% by mass, and Mn: 0.50 to 1.15% by mass as additive elements, wherein a mass ratio of the C to the Si is 0.30 to 0.75, a mass ratio of the C to the Mn is 0.30 to 0.75, and a mass ratio of the Si to the Mn is 0.70 to 1.60, and a Vickers hardness of the shot is HV 400 to
 800. 2: The shot according to claim 1, wherein the total content of the C, the Si, and the Mn is 1.80 to 2.40% by mass. 3: The shot according to claim 1, wherein the iron-based alloy further contains 0.30 to 1.0% by mass of at least one element selected from the group consisting of Cr, Ni, Cu, Mo, Al, B, V, Nb, and Ti. 4: The shot according to claim 1, wherein the shot is substantially formed from a tempered martensite phase. 5: The shot according to claim 1, wherein the number of particles having blowholes is 5% or less of the entire shot. 6: The shot according to claim 1, wherein, when a length of the particle in a longitudinal direction is designated as L and a maximum diameter in a direction perpendicular to the longitudinal direction is designated as S, the number of the particles in which L/S is 2.0 or more is 5% or less of the entire shot. 7: The shot according to claim 2, wherein the iron-based alloy further contains 0.30 to 1.0% by mass of at least one element selected from the group consisting of Cr, Ni, Cu, Mo, Al, B, V, Nb, and Ti. 8: The shot according to claim 2, wherein the shot is substantially formed from a tempered martensite phase. 9: The shot according to claim 3, wherein the shot is substantially formed from a tempered martensite phase. 10: The shot according to claim 2, wherein the number of particles having blowholes is 5% or less of the entire shot. 11: The shot according to claim 3, wherein the number of particles having blowholes is 5% or less of the entire shot. 12: The shot according to claim 4, wherein the number of particles having blowholes is 5% or less of the entire shot. 13: The shot according to claim 2, wherein, when a length of the particle in a longitudinal direction is designated as L and a maximum diameter in a direction perpendicular to the longitudinal direction is designated as S, the number of the particles in which L/S is 2.0 or more is 5% or less of the entire shot. 14: The shot according to claim 3, wherein, when a length of the particle in a longitudinal direction is designated as L and a maximum diameter in a direction perpendicular to the longitudinal direction is designated as S, the number of the particles in which L/S is 2.0 or more is 5% or less of the entire shot. 15: The shot according to claim 4, wherein, when a length of the particle in a longitudinal direction is designated as L and a maximum diameter in a direction perpendicular to the longitudinal direction is designated as S, the number of the particles in which L/S is 2.0 or more is 5% or less of the entire shot. 16: The shot according to claim 5, wherein, when a length of the particle in a longitudinal direction is designated as L and a maximum diameter in a direction perpendicular to the longitudinal direction is designated as S, the number of the particles in which L/S is 2.0 or more is 5% or less of the entire shot. 17: The shot according to claim 1, containing particles with the average particle size of 0.1 to 1.5 mm. 18: The shot according to claim 1, wherein the iron-based alloy contains C: 0.30 to 0.45% by mass, Si: 0.55 to 1.05% by mass, and Mn: 0.55 to 1.00% by mass. 19: The shot according to claim 1, wherein the mass ratio of the C to the Si is 0.35 to 0.60, the mass ratio of the C to the Mn is 0.35 to 0.60, and the mass ratio of the Si to the Mn is 0.80 to 1.40. 20: The shot according to claim 1, wherein the total content of the C, the Si, and the Mn is 1.85 to 2.25% by mass. 